The wind propulsor type called ‘Flettner Rotor’ is a mechanically operated cylindrical sail installed vertically on the deck of the ship and rotates at certain speed range.
The name comes from the German aviation engineer and inventor Anton Flettner who experimented on the idea of a rotating cylinder generating lift force inside an air stream, tested in both marine and aircraft applications during the 1920’s.
Rotor Sails are a modernized version of the Flettner Rotors and are considered as mechanical sails, currently installed and operating in RoRo, RoPax, product tanker and bulk carrier ships.
Main Components
- The Rotor Sail is mainly comprised of the below main components (Figure 3.1):
- The cylindrical Rotor
- The rotor Tower
- The Foundation with the bearings
- The Drive system (electric variable speed)
- The wind sensors
- The Power Control and Bridge operating panel
Flettner rotor on the Viking Grace / Source: D. Newman
The rotating cylinder is usually made of a laminated glass fibre and carbon fibre composite material.
The Rotors are utilizing a physical phenomenon called ‘Magnus effect’. When the rotor is rotating, a small boundary layer is formed around it, inducing a pressure differential between its opposite sides when it is exposed to an air flow (wind).
A resultant perpendicular to the wind flow force is generated and the maximum exploitation of the wind happens in beam reaching directions to the ship. The Rotor sail speed of rotation increases or reduces the thrust force magnitude.
The Rotors consume electrical power from the existing ship’s electric grid to rotate at their specified operating speed, usually 0 to 180 or 250 rpm. The power required for the rotor rotation can vary depending on the size of the rotor sail, the aspect ratio and the max RPM, ranging from 40 kW up to 160 kW. Usual dimensions of rotors are 18 to 35 m tall and 2 to 5 m in diameter.
The Rotor Sail normally operate in fully automatic mode, in a way that the operation starts and stops from the Bridge control panel. When the wind conditions are unfavourable then the Rotor Sail can either rotate more slowly to mitigate unfavourable drag or completely stop.
Some Rotor Sails can be tilted to decrease the air draught or moved along rails on deck to facilitate cargo handling operations.
Types of Flettner rotor (FR) / Source: D. Newman
When placing multiple Rotor Sails on deck, as far as practical and as far away from each other, with a general rule of thumb being that each Rotor should be located such that there is a minimum distance equalling the height of the rotor between adjacent rotor sails and nearby deck structures.
To install a Rotor Sail on deck, a foundation is needed to incorporate the housing needed for the main driver system and the lower bearings. A power supply and the remote-control panel on the bridge are required and auxiliary power to enable tilting-moving if necessary.
The material of the Rotor Sail is usually composite, thus the expected lifetime depending on the manufacturer material quality choices could be at or above 5 years, with some manufacturers reporting that with good quality material the design lifetime of the rotor composite can reach even up to 20 years.
– Information courtesy of Konstantinos Fakiolas’ book ‘Wind Propulsion Principles’, Edition 1 –